Eicosapentaenoic acid (hereinafter abbreviated as "EPA") has been known to be effective in prevention and therapy of thrombotic maladies as it properly controls the content of cholesterol in blood in a living body and has an effect to prevent thrombotic disturbances.
The fish oils containing EPA can be obtained as by-products in producing such products as low-fat fish meal and fish cakes in oil expression by the method such as boiling or expressing method, from such fish as sardine and/or pilchard, chub mackerel, pacific saury, alaska pollack, etc. Thus the oil-expressing method employed in obtaining fish oils is quite rude in itself, and it commonly invites lowering of freshness of the material before oil-expression and, furthermore, formation of low-molecular amines through decomposition of proteinaceous materials in the fish meal in oil-expressing so that mingling of the amines in the fish oil cannot be avoided. On the other hand, fish oil contains a large amount of highly unsaturated fatty acids in addition to EPA, and these highly unsaturated fatty acids are oxidatively decomposed during the storage, which unavoidably form low molecular acids and/or low molecular ketones or aldehydes. Therefore, even though the fish oil freshly expressed from fresh materials has no perceptible odor, the low molecular amines contained in the fish oil is reacted, with the lapse of time, with the low molecular ketones and/or aldehydes that are formed during the storage and thereby odorous substances are formed, which give forth a nasty smell and cause lowering of the commercial value of said fish oil.
In order to prevent emission of such fish-oil-odors, they have conventionally employed the methods to subject fish oil expressed to refining treatments such as deacidification, deodorization and the like to remove impurities therefrom.
However, if these conventional refining methods could remove the odorous substances, it was still impossible to remove completely amines and/or ketones, the precursors of the odorous substances. Thus even though the refined fish oil obtained thereby has no odors right after the refinement, fishy odors come to be emitted with the lapse of time, as the odorous substances are formed. And there is a tendency that emission of fishy odor becomes more remarkable as the refined fish oil contains more higher content of highly unsaturated fatty acids such as EPA and the like. Therefore, no fish oils having higher concentration of EPA, although being odorless after a lengthy period of storage are available till to now.